Pregnancy Due Date Calculator

Estimate pregnancy timelines, gestational age, and schedule common prenatal screening tests.

Pregnancy timeline and due date calculations

Estimated Due Dates (EDD) are typically projected using Naegele's Rule, which assumes a baseline of 280 days from the first day of the Last Menstrual Period (LMP). Dating scans using ultrasound imaging calculate gestational age by measuring crown-rump lengths in early weeks to verify the timeline.

IVF embryo transfer calculations adjust based on day count: Day-3 transfers subtract 17 days, while Day-5 transfers subtract 19 days to establish equivalent LMP baselines. Standard trimesters subdivide development into 13-week milestones.

Built and maintained by Meet Shah · Last updated 19 July 2026

What this tool is used for

Getting an estimated due date from a last period date.
Working out gestational age in weeks for a form.
Seeing which trimester a particular date falls in for planning leave.
Getting a date to discuss at an appointment.
Comparing an LMP-based estimate against a conception-based one.

Frequently Asked Questions

How is a due date calculated?: By Naegele's rule: last menstrual period plus 280 days (40 weeks). Note this counts from the LMP, not from conception — so at '8 weeks pregnant' the embryo is about 6 weeks old, which surprises almost everyone.
How accurate is it?: Only about 4% of births occur on the due date, and roughly 80% fall within two weeks either side. It is a midpoint estimate, not a prediction — which is why some clinicians prefer to give a due WEEK.
What if my cycle is not 28 days?: Naegele's rule assumes a 28-day cycle with ovulation on day 14. A longer cycle shifts ovulation later, so the LMP-based date runs early — hence the adjustment of adding the difference between your cycle length and 28.
Why does an ultrasound date override the LMP date?: Because first-trimester crown-rump measurement is more accurate than a recalled LMP, which depends on memory and on regular cycles. Guidelines say to redate if the two differ by more than about 5-7 days in the first trimester.
Is this medical advice?: No — it is arithmetic on the dates you enter. Dating in a real pregnancy is established by a clinician, usually with an ultrasound, and any decision about a pregnancy should rest on that rather than on a calculator.
Why is a pregnancy dated from the last period rather than conception?: Because the last period is a date most people know and conception usually is not. It builds in roughly two weeks before conception occurred, which is why "weeks pregnant" is about two weeks ahead of the embryo's actual age.
How is a due date adjusted for IVF?: From the transfer date and the embryo's age, since conception is known precisely — which makes it more reliable than either LMP or a scan. The usual LMP formula does not apply and will be off.

Common errors and gotchas

Treating the date as a prediction, when it is an estimate and most births are not on it.
Assuming a 28-day cycle when yours differs, which shifts the estimate.
Using a conception date that was itself estimated, which compounds the uncertainty.
Preferring the calculation over a dating scan, which is more accurate.
Treating the result as medical advice rather than a rough date.